The interactions between the malignant plasma cells of multiple myeloma (MM) and the bone marrow stroma are essential for myeloma cell proliferation and survival, and play a central role in mediating resistance to chemotherapy. Defining the molecular and cellular elements involved in these interactions has been difficult due to the complexity of the bone marrow microenvironment. Our approach has been to identify cell surface receptors that are characteristically expressed on myeloma cells, and then identify cells that express the ligands for these receptors. We have focused on the costimulatory receptor CD28, which is expressed predominantly on T cells but also on normal plasma cells and myeloma cells. For T cells, CD28 is activated upon binding to CD80 (B7-1) and CD86 (B7-2) expressed on antigen presenting cells (especially dendritic cells (DC)), and in conjunction with T cell receptor signaling significantly augments T cell proliferation, effector function and survival. In contrast to T cells, little is known about the function of CD28 in plasma/myeloma cells. However, clinical studies have demonstrated a highly significant correlation between myeloma disease progression and myeloma cell CD28 expression, and MM CD28 expression as a significant predictor of shortened disease-free survival in newly diagnosed patients. These clinical observations implicate an important contribution of CD28 to myeloma cell survival, particularly under chemotherapy treatment selection pressure. Our preliminary studies demonstrate that direct activation of myeloma cell CD28 induces activation of NF?B, downregulates MM cell proliferation and protects against serum starvation and dexamethasone-induced death. Coculture with dendritic cells (DC) expressing CD80/CD86 also elicits CD28-mediated effects on MM survival and proliferation. Furthermore, we have found that CD28+/CD86+ myeloma cell lines express indoleamine 2, 3 dioxygenase (IDO), a tryptophan-catabolizing enzyme that has been previously shown to be induced by B7 crosslinking in DC to mediate T cell anergy/unresponsiveness. We hypothesize that myeloma CD28 transduces survival signals through downstream activation of NF?B, and that MM CD28 is activated by a cell-cell interaction with DC (and/or other CD86+ myeloma cells) located in the bone marrow. Furthermore, "back" signaling through B7 (on the myeloma cell or DC) induces IDO and contributes to the suppression of cell-mediated immunity characteristically seen in MM patients. The Aims of this application are 1). Define the molecular components of CD28 activation in myeloma cells, 2). Characterize the cellular responses to CD28 activation, 3). Define the cellular partners that activate myeloma CD28, 4).Assess if targeting myeloma CD28 itself, components of its signaling pathway or stromal partners can be exploited therapeutically. These studies may provide novel strategies for the treatment of multiple myeloma.